Measles remains a major worldwide health problem. The primary complications leading to morbidity and mortality are caused by secondary infections and the autoimmune disease post-infectious encephalomyelitis likely caused by the suppressive effect of measles virus on the immune system. The investigators propose to identify the viral and host determinants of the immune suppression caused by measles infection. The areas to investigate are related to those that have been implicated in the regulation of immune suppression. The specific aims are: 1) to determine whether activation-induced T-cell death occurs as a consequence of the immune response to measles virus; 2) to determine further the type 2 nature of the immune response to measles virus; 3) to determine the effect of measles virus infection on the function of stromal epithelial cells and the mechanisms by which measles infection of thymic stromal cells induces apoptosis of thymocytes: 4) to determine the role of measles virus strain on the outcome of thymic stromal cells and the molecular and biologic basis of these differences in virulence; and 5) to determine the immunologic consequences of primary and secondary routine measles immunization on the immune responses of children of different ages.